Swine production today can be represented by a multilevel pyramid, with certain offspring at each level used in the next lower level for breeding. The top level of the pyramid is the genetic nucleus (GN). The next levels from top to bottom are generally the daughter nucleus (DN), the multiplier, or multiplication unit, and finally the commercial level, generally comprising commercial farms where slaughter pigs are being produced, respectively.
Typically, genetic progress of a line takes place in the pure line population at the genetic nucleus. The GN animals will have relatives at lower levels of the pyramid, pure bred as well as crossbred. Trait data collected from these relatives contribute to the estimation of the genetic merit of GN animals. Within a line at the GN, once selected, parents that produce the next generation are in general randomly mated with one another, while avoiding matings between closely related individuals, with the goal of increasing the genetic merit of the next generation. An increase in the genetic merit of the next generation constitutes genetic progress. An increase in genetic merit, in this context, means that for a given trait or set of traits, the individuals in the successive generation will express the desired trait or set of traits more strongly than their parents. With respect to undesirable traits, an increase in genetic merit means the individuals in the successive generation will express the trait or set of traits less strongly than their parents.
Genetic change, including desirable genetic change (i.e., genetic progress per year), (“dG”) can be measured as the difference between the average genetic level of all progeny born in one year and all progeny born the following year. The difference is the result of selected parents having higher genetic merit than the average genetic merit of all the selection candidates (the animals available for selection). In ideal conditions, this depends upon the heritability (h2) of the trait and the difference between the average performance of selected parents and that of selection candidates. The heritability of a trait (h2) is the proportion of observable differences (phenotypic variance, σ2P) in a trait between individuals within a population that is due to additive genetic (A), as opposed to environmental (E), differences (h2=σ2A/σ2P=σ2A/(σ2A+σ2E)). The difference between the average performance of selected parents and that of all selection candidates (of which the selected parents are a subset) is also known as the selection differential.
The genetic progress per year is the result of genetic superiority of selected males and of selected females. This is expressed in the following equation:dG={(RIH*i)males+(RIH*i)females}*σH/(Lmales+Lfemales),Where, R=the accuracy of selection, i=the selection intensity, σH=genetic variation and L=generation interval, for male or female parents.H=breeding goal that combines genetic merit (g) of the traits (1 to m) that need to be produced weighted by the economic values (v) of the traits (H=v1g1+v2g2+ . . . +vmgm). The economic value is positive if selection is for larger phenotypic values and negative if selection is for smaller phenotypic values.I=an index that combines all the trait information on the individual and its relatives and is the best estimate of the value of H for the individual.
Selection is more effective when non-genetic effects are removed (e.g. by comparing each performance record to the average of the contemporary group) and when information from relatives is used in addition to that of the animal itself. This is achieved through the computation of estimated breeding values (EBVs) using for instance multiple trait BLUP methods. Environmental factors such as HYS (herd-year-season) are in the model to correct for environmental effects and simultaneously information from relatives is included through the use of the relationship matrix. More trait information from more relatives results in a higher accuracy (RIH) of the EBV.
In a large population, the selection intensity depends upon how many animals are tested and how many are selected—the lower the proportion selected the higher the selection intensity and the larger the genetic progress, all else being equal. Thus, in order to maximize genetic progress, one should rank all tested animals based on the EBV and then select the minimum number of top boars and sows required to maintain the line, breed and/or herd size and to avoid inbreeding problems. This ensures that the average EBV of selected animals is substantially higher than the average EBV of all animals tested. In particular through the use of artificial insemination (AI), one needs to select fewer boars than gilts and the selection intensity for males is higher than for females.
The generation interval for males (or females) is the average age of male parents (or female parents) when progeny are born. In general sows produce more than one litter at the GN and the L for females tends to be larger than the L for males.
The annual rate of genetic progress depends on the generation interval and on the superiority of the parent's EBVs compared to that of the selection candidates. In general, males contribute more to the genetic progress per year than the females.
Examples of important traits in the swine industry are feed efficiency, i.e., a measure of an animal's efficiency in converting feed mass into increased body mass (also known as feed conversion or feed to gain ratio), and average daily gain, i.e., the average daily weight gain for an animal. Traits are measured in different units (e.g., number of pigs, pounds per day, inches, etc.), are not of equal economic importance in all global markets, and are not genetically influenced to the same degree (i.e., different heritability coefficients). Generally speaking, production traits such as feed efficiency and average daily gain have high heritability. In contrast, reproductive traits such as fertility and litter size generally have low heritability.
There is a need in the swine industry to increase the rate of genetic progress in lines as well as to lower operational costs on breeding and commercial swine farms.